The objective is to develop an assay protocol for monitoring human populations to determine whether they have been exposed to mutagenic agents or, in instances where exposure is known, to determine if their exposure to known mutagens is at a sufficient level to produce a biological effect that can be detected as a increase in the frequency of somatic cell mutants. The protocol will be based on the combined use of short and longer term assays for mutation in peripheral blood lymphocytes at the hypoxanthine guanine phosphoribosyl transferase gene locus.